The rock cycle metamorphic rocks

EARTH MATERIALS VIII

The Rock Cycle: Metamorphic Rocks

Professor Peter Doyle




THE ROCK CYCLE


Metamorphic rocks: typically complex

Pressure indicated by fabric (foliation)

Heat indicated by growth of new
minerals/crystals


Metamorphic Rocks
Two major types of metamorphism
• Regional metamorphism occurs when
large areas of the crust are subjected to
high temperatures and/or pressures
• Contact metamorphism occurs adjacent
to igneous intrusions causing mainly an
increase in temperature but also pressure
in some instances

METAMORPHIC FACIES: temperature & pressure regimes

Low Pressure

High Pressure
Low temperature

High Temperature

Assemblage of metamorphic rocks formed under a similar range
of pressure and temperature conditions


METAMORPHIC FACIES: typical settings
Intraplate: normal range
Contact: High
temperature

Arc: increased
temperature

Subduction zones: high pressure


Increasing Temperature

METAMORPHIC FACIES: mineral assemblages

Original chemistry of parent rock controls the mineral
assemblage formed during metamorphism

Metamorphic Index minerals
• INDEX MINERALS are diagnostic minerals formed by
metamorphic reactions
• They are a function of P, T and rock & fluid
compositions (X) at time of the reaction
• Used to define metamorphic zones
• Demonstrates progress of metamorphism
REGIONAL METAMORPHISM OF SHALES
LOW GRADE

HIGH GRADE

CHLORITE
BIOTITE
GARNET
STAUROLITE
KYANITE
SILLIMANITE

Index Minerals
formed: controlled
by original rock
(X), P, and T.


ISOGRADS: lines on a geological map showing locations
of first appearance of index minerals

Increasing grade

Record of sets of P-T-X conditions in the metamorphic
reaction


Minerals function of temperature, pressure & parent rock
Increasing Grade & relevant facies

Index
minerals

REGIONAL METAMORPHISM OF SHALES


Increasing Grade & relevant facies

Index
minerals

REGIONAL METAMORPHISM OF BASALTS


CONTACT METAMORPHISM: heat alone

SANDSTONES & SHALES:
(siliciclastic source)

LIMESTONES: (carbonate source)

Minerals produced a function of temperature & parent rock


TEXTURES: INDICATORS OF METAMORPHIC
PROCESS

Pressurized

Heated


Foliated rocks: pressurised rocks
• Pressure induces re-organisation and growth of
new minerals
• Large percentages of platey minerals
• Increasing grade indicated by level of foliation:
Slate-Phyllite-Schist-Gneiss
• Slate: splitting surfaces at an angle to bedding
• Phyllite:
Phyllite similar, with platy mineral growth
• Schist: segregation of platey minerals in preferred
orientation
• Gneiss: Coarser grade foliation, distinct banding


FOLIATED ROCKS

phyllite

FOLIATION: INTENSITY INCREASES WITH INCREASING

PRESSURE


DEVELOPMENT OF FOLIATION

Platy minerals oriented normal to compressive forces


Compressive
stress

GRAIN
ORIENTATION

Schist


SLATEY CLEAVAGE: planar foliation (not bedding)
Little new mineral growth


SCHISTOSITY: wavy foliation


Porphyroblasts:
new crystal
growth in matrix
SCHIST WITH GARNET PORPHYROBLASTS

GLAUCOPHANE BLUE SCHIST

High Pressure metamorphism


GNEISSOSE FOLIATION, SEGREGATION OF
MINERALS (micas, feldspar, quartz, amphibole,
etc)


Non-foliated metamorphic rocks I:
Contact metamorphism
• Formed from contact heating or regional
metamorphism
• Hornfels: formed by high temperature/low
pressure contact metamorphism, uniform
grain size with no preferred orientation
• FINE GRAINED GRANULAR (GRANOBLASTIC)

• Spotted Hornfels: similar, but with small
irregular porphyroblasts of minerals such as
cordierite & andalusite
• Minerals form during metamorphic heating
at low pressures


Contact metamorphic rocks
Spotted hornfels:
porphyroblasts

Hornfels: uniformly granular


Non-foliated metamorphic rocks II:
Regional metamorphism
• Quartzites: metamorphosed quartzrich sandstones
• metamorphism produces recrystallised,
recrystallised
uniform-sized,
uniform-sized quartz crystals with no
foliation
• Marbles: metamorphosed limestones
• metamorphism produces uniform,
uniform
equigranular,
equigranular interlocking recrystallised
calcite crystals with no foliation.